Not Just 'Live Rock,' Moving Rock...

In January of 2004 I was busily readying my newest (and biggest to date) reef tank, a standard 75-gallon all glass aquarium (AGA). My existing reef was a simple 29-gallon that had been with me for many years, and was just no longer large enough to allow me to create the environment I desired in my little glass ocean. Trying to do everything right prior to transferring the contents of the soon-to-be obsolete 29-gallon tank, I had prepared the 75 with a five to six inch sandbed and over 20 pounds of live rock to start things off. While taking a pause for budgetary constraints, I entertained myself by watching "pods" in the new 75 (and also in the established 29). I find that no matter how much I have spent, in time or money, or in acquiring really interesting fish and corals, that the most fascinating animals in those tanks are still the tiny creatures that arrive in the tank largely by accident.

After the first night, I was certain I had inherited a mantis or pistol shrimp, but I wasn't certain which; I had only had Tim, a G. smithii mantis living in a 10 gallon nano tank, for a couple months at that time and was still learning to distinguish his clicks from the pistol shrimp pops. Unfortunately, my hectic schedule left me little time for working in or even looking at the tank that week. In fact, I was unable to watch for the possible mantis shrimp until late that following evening, just before lights out. What I noticed almost immediately was a single piece of rubble, half the size of a grown man's fist, far out of place.

Having never in all my reefkeeping days encountered anything that could move a piece of rock that size from one end of the tank to the other (48" in this case), I did what any sane man in my place would do: I accused the wife of fiddling around in the tank. After getting the accusation out of the way, and accepting a little more ribbing than was, in my opinion, necessary, I was just going to let it go as an odd occurrence and keep an eye out for something large that surely must be in the rubble. I was still in for a surprise. As I reached into the tank to move the rock back to the pile, the rock suddenly lurched forward and seemed to 'hunker down' before I could get to it. So, I immediately withdrew my hand and reached for the net, hoping to capture the mantis shrimp which I thought I had stumbled upon (I don't think I really 'jerked' my hand away, but my wife tells the story a little differently). Before netting my prey I got down really close to the glass to check out the fearsome mantis. As it turns out, it wasn't a mantis shrimp after all, but instead the strangest crab I had ever seen.

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Being the know-it-all that I am, I of course immediately knew what it was although I thought I'd challenge one of the resident experts at Reef Central to ID the creature for confirmation (I had no idea of its exact identity and still think I've managed to place it only in the proper genus). Already having a strong rapport with Dr. Ron Shimek from previous ID questions (as I fancy myself a better macro-photographer than I have any reason to believe I am), I decided to ask him for an ID.

The good Dr. Shimek played it safe, of course, and informed me that the rock could have been moved by any number of strange creatures:

  • It could have been a large polychaete or eunicid worm, or...

  • The rock could have been moved by a fish that managed to hitchhike in, even though no inhabitants had been added yet.

In either of the above scenarios it is possible that the crab was just hanging on for the ride. His final suggestion was that the crab might be a member of the Family Dromaiidae. This was the most shocking assertion, of course, because even I would know if there were an Emu in my tank!!

After having a nice chuckle (at the good doctor's expense - he was a great sport about it), Dr. Shimek corrected a typographic error he'd made and placed the crab into its rightful spot as a member of the Family Dromiidae.

A fellow member of Reef Central, SDM, recently prompted me to take a closer look at the crab again to further attempt classification down into a genus under the Family Dromiidae. To that end he'd made much ado about this crab (that I affectionately refer to as 'StrongBad' or "SB") being one of the 'decorator crabs' or "sponge-back crabs" now being sold into the trade with more frequency.

Of the first three genera under the Family Dromiidae, Cryptodromiopsis, Dromia and Dromidia, all of the species mentioned were described as decorator crabs to some degree or another. These descriptions made no references to the crabs carrying rocks around on their backs nor to the specialized legs required for holding onto such a rock.

The fourth genus however, Hypoconcha, showed much promise. The animals in this crab genus are described as having the fourth and fifth sets of walking legs highly modified to hold shells on their backs. This peculiar habit has earned them the common name "shell-backs" in one of the referenced articles below (for which I would never have searched since "SB" has had to suffer through carrying a piece of rock around and sculpting the underside smooth through long hours of chiseling work).

While I have yet to narrow the classification down to the species level, I don't really feel the need to delve more deeply at this time. Additionally, this is an amateur endeavor at best, and I have neither the resources, nor the tools, nor the skills to do the job. Besides, at any moment now, someone else who is much smarter and more attentive to details than I am may come along and prove me wrong in less time than it took me to compose this article.

Additionally, I have been unable to determine what the little crab is eating, even after eight months of "care." I can tell you he resists offerings of shrimp pellets or frozen fare. Most of his time is spent moving about the tank using his pinchers to shovel around in the sand. I worried about his specific diet for the first month or so, after it became clear that Tim, the G. smithii mantis with whom he was sharing the tank, would not consume him. Since that time, though, I have stopped worrying. The tiny 10-gallon tank which houses my is teeming with "pods" of various shape and size and worms of at least as many different types. I've noted that he has molted three times while in my care and still fits into the original "'shell" of his choosing. The total width of his carapace is approximately ¾".

It takes roughly two seconds for him to flip over his home should he lose his grip on the rock-work (or whenever I feel the need to see him again and decide to turn him over for viewing). It takes much longer for him to build up the will to expose himself and right his overturned home.

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Tucked away and unhappy at being overturned.
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Gathering up some nerve…
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* shuffle *
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* scamper *
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* slide *
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* DASH!! *
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* flip *

Dr. Ron Shimek has been kind enough to provide some images of a related crab shot in the wild. I have included them below with his captions included beneath each image.

I found this shell beside the bottom of a patch reef. Note the trail, much as in Scott's image.
Curious, I flipped the shell over. Note the crab, tightly retracted.
After I backed off, the crab started to flip itself.
Still flipping...
Back where it started.

This is certainly not an all-inclusive or scientific look at the genus, but instead an interesting reminder of the incredible diversity (and strangeness) of life on the reefs, both in the wild and by proxy within our little glass boxes. I wouldn't recommend running out to request strange hitchhikers for your reefs without first performing some research. There are several reasons I've refrained from adding him to my display reef. He has a tendency to 'bulldoze' over corals and live rock; I have no idea what he might prefer to eat in the reef that he may be substituting in the mantis-nano. Besides, I would rarely see the little guy anyway. All that having been said, nano-tanks devoted to a specific creature or group of creatures (Ninja-crustaceans and meals-for-Tim, in this case) are a great source of entertainment and learning at a cost greatly reduced compared to that of our main display tanks. It's getting a little crowded in the crab-nano tank these days. I can't wait to find something new so I can start another nano-tank!


Original telling and taxonomic start given by Dr. Ron Shimek

Integrated Taxonomic Information System; Taxonomic Hierarchy

Southeastern Regional Taxonomic Center; Species of the Month

Guinot D. & Tavares M. 2003. - A new subfamilial arrangement for the Dromiidae de Haan, 1833, with diagnoses and descriptions of new genera and species (Crustacea, Decapoda, Brachyura). Zoosystema 25 (1) : 43-129.

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Not Just 'Live Rock,' Moving Rock...By Scott Chevy -